Earthquake Fault Systems Research Articles

On the geometry of an earthquake fault system

Invariants of a two-point correlation function of the seismic moment tensor were used to investigate an earthquake fault system. The geometry of the fault system is significantly different from the standard model of an earthquake fault, i.e. coherent rupture on a planar surface. Contrary to the ‘flat-fault’ model, we see clear evidence for non-planarity of the fault system geometry, and observe that the focal mechanisms of neighboring events may have very different orientation, i.e. they undergo large three-dimensional rotations. Therefore, earthquake deformation models need to be fully three-dimensional and should include large rotations. The spatial behavior of the invariants is approximately the same for earthquakes in different depth intervals: shallow, intermediate, and deep. The temporal behavior of the invariants differs only in that shallow earthquakes are clustered in time, whereas for deeper events the clustering is much less pronounced; as soon as we ‘decluster’ shallow seismicity, the invariants' temporal properties become similar for earthquakes of all depths. This demonstrates that the basic geometrical properties of earthquake rupture do not depend on depth, and therefore they are generally independent of rheological properties of rocks, lithostatic pressure, or the presence of a free boundary for strong shallow earthquakes.

断層の幾何学的バリヤと破壊プロセス

Geometric patterns of surface earthquake faults are closely related to rupture propagation processes especially in the case of strike-slip faults. In this paper, the author examined geometric features of four strike-slip earthquake faults in Japan, that is, the Kita-Izu earthquake fault system of 1930, Izu-Oshima Kinkai earthquake fault system of 1978, the Nobi earthquake fault system of 1891 and the Kita-Tango earthquake fault system of 1927. Bends or jogs along a fault can be called as geometric barriers, which are classified into two categories as extensional and compressional, from a view point of volumetric change (Fig. 1). Studies of earthquake surface faults of recent inland large earthquakes in Japan show that ruptures were initiated from compressional barriers and terminated at extensional barriers. A compressional barrier in the course of rupture propagation could be the position of next rupture nucleation. Compressional barriers can be commonly described as “fragmentation barriers” (King, 1987) because short faults of various orientation are very popular in and around compressional barriers. The location of severe damage area is also closely related to that of barrier.Study of geometric characteristics of active faults will become very important because they give us a key idea on rupture initiation and termination as in the case of surface earthquake faults. Recognition of barriers along an active fault system from geometrical investigation will provide us some locations of detailed observation for geodetical, geophysical and geochemical anomalies along precautious faults.

Stochastic model of earthquake fault geometry

Summary An earthquake fault pattern is assumed to consist of a system of infinitesimal, elementary dislocation loops. After an initial dislocation, subsequent ruptures occur according to a critical branching process. The position of each secondary dislocation loop is randomly shifted, along the initial fault-plane, from the location of the main shock. In addition, the orientations of the fault-plane and slip vector of the secondary dislocations are rotated according to a three-dimensional Cauchy distribution which has one intrinsic scaling parameter. In their turn, the secondary dislocations produce new dependent shocks according to the same law; the process cascades indefinitely. We simulate an earthquake fault system by the Monte Carlo method. This fault pattern is both visually and statistically similar to real earthquake faults. The model yields statistical spatial distributions that correspond to those of the analysis of the catalogue data of real earthquakes. We incorporate in our scheme the time-magnitude model of earthquake occurrence developed earlier. By the Monte Carlo method we then simulate synthetic sequences that reproduce all known magnitude-space-time statistical properties of real earthquake catalogues. The results of these simulations make us question the suitability of some terms that are commonly used in the theory of an earthquake source. For example, as a consequence of the validity of the stochastic model, the concepts like ‘an individual earthquake’, ‘fault-plane’ or ‘fault-surface’, ‘crack-tip’ or ‘fault-tip’, and ‘friction’, do not have an unambiguous definition.